Three of the following four number pairs are alike in a certain way and one is different. Pick the odd one out.

56 : 136

78 : 127 = 127 - 78 = 49  (it is perfect square of 7)

61 : 97  =  97 - 61 = 36 (it is perfect square of 6)

56 : 136 =  136 - 56 = 80  (it is not  perfect square ) Odd

25 : 89 = 89 - 25 = 64 (it is perfect square of 8)